The present invention relates to selectively improving the material properties of structural members and, more particularly, relates to selectively refining the grain structure of structural members.
Conventional structural assemblies, such as those used in the manufacture of military and commercial aircraft, are commonly fabricated from a plurality of structural members secured together to form a built-up structure. The structural members are typically forged, machined from stock material or cast in various configurations from steel, stainless steel, magnesium, magnesium alloys, copper, copper alloys, brass, aluminum, aluminum alloys, or titanium.
During use, aircraft structural assemblies are subjected to static and cyclic loads, as well as a variety of environmental conditions, temperature variations, and severe acoustic and vibration environments, all of which create mechanical and thermal stresses. While these operational stresses generally exist throughout the individual structural members forming the structural assembly, certain regions of each structural member are typically subjected to comparatively higher magnitudes of stress. For example, under cyclic loading conditions, threaded openings machined into a structural member to facilitate attachment to other structural members when forming a structural assembly can significantly increase the stress in the immediate vicinity of the opening. High operational stresses can lead to micro-cracking or fracture of the structural members of a structural assembly, which can result in the eventual failure of the assembly. In addition, due to the large number of parts and fasteners utilized in the construction of conventional structural assemblies, maintenance, repair and replacement of structural members, if necessary, can be time consuming and labor intensive, which can be costly over the life of the assembly.
In seeking to enhance the strength, toughness and fatigue resistance of structural members and, thus, increase the useful life of structural assemblies, designers have modified the dimensions of the structural members in the regions of high operational stress, for example, by increasing the thickness of the members in these regions. Designers have also experimented with substituting more exotic and, typically, more expensive types of materials for use in the fabrication of the structural members. Structural members can also undergo precipitation hardening whereby the members are solution heat treated and then aged at predetermined temperature schedules to thereby improve the grain structure and, thus, the material properties of the members. However, the precipitation hardening process can be time and labor intensive and provides only limited improvement of material properties, and even selective increases in the thickness of a structural member can negatively increase the overall weight of the structural assembly, as well as resulting in increased material cost.
Accordingly, there remains a need for improved structural members and methods of manufacture that will increase the operational life of structural assemblies. The improved structural members must have enhanced strength, toughness and fatigue resistance, especially in those regions subjected to high operational stresses.
The present invention provides a structural member defining a first region characterized by comparatively high operational stress and a second region having a more refined grain structure than other portions of the structural member positioned such that the second region at least partially encompasses the first region to thereby selectively improve the strength, toughness and fatigue resistance of the structural member in the first region. The structural member may be formed from steel, stainless steel, magnesium, magnesium-based alloys, brass, copper, beryllium, beryllium-copper alloys, aluminum, aluminum-based alloys, aluminum-zinc alloys, aluminum-copper alloys, aluminum-lithium alloys, or titanium.
The second region can be defined based upon the particular region that will be subjected to comparatively high operational stress. For example, the structural member may define a threaded opening at least partially contained within the second region. Alternatively, the structural member can have an I-shaped configuration having opposed end portions and a web interconnecting the end portions, wherein the second region encompasses at least a portion of the web of the I-shaped member. In another embodiment, the structural member has an I-shaped configuration wherein said second region includes at least a portion of at least one of said opposed end portions. In yet another embodiment, the structural member has a tubular configuration. In still another embodiment, the structural member defines a plurality of regions having refined grain structures, wherein the regions are spaced apart and generally parallel. In still another embodiment, the structural member defines a first set of regions having refined grain structures and a second set of regions having refined grain structures. The first set of regions are spaced apart and generally parallel. The second set of regions are spaced apart and generally parallel and wherein the first set of regions intersects the second set of regions to thereby define a plurality of containment zones.
The present invention provides a structural assembly including a plurality of structural members. The plurality of structural members are secured together to form the structural assembly. The structural members may be formed from steel, stainless steel, magnesium, magnesium-based alloys, brass, copper, beryllium, beryllium-copper alloys, aluminum, aluminum-based alloys, aluminum-zinc alloys, aluminum-copper alloys, aluminum-lithium alloys, or titanium. At least one of the plurality of structural members defines a first region characterized by comparatively high operational stress and a second region having a more refined grain structure than other portions of the structural member positioned such that the second region at least partially encompasses the first region to thereby selectively improve the strength, toughness and fatigue resistance of the at least one structural member in the first region.
The second region can be defined based upon the particular region that will be subjected to comparatively high operational stress. For example, the at least one structural member may define a threaded opening at least partially contained within the second region. Alternatively, the at least one structural member can have an I-shaped configuration having opposed end portions and a web interconnecting the end portions, wherein the second region encompasses at least a portion of the web of the I-shaped member. In another embodiment, the structural assembly has an I-shaped configuration wherein said second region includes at least a portion of at least one of said opposed end portions. In yet another embodiment, the at least one structural member has a tubular configuration. In still another embodiment, the at least one structural member defines a plurality of regions having refined grain structures, wherein the regions are spaced apart and generally parallel. In still another embodiment, the at least one structural member defines a first set of regions having refined grain structures and a second set of regions having refined grain structures. The first set of regions are spaced apart and generally parallel. The second set of regions are spaced apart and generally parallel and wherein the first set of regions intersects the second set of regions to thereby define a plurality of containment zones.
The present invention also provides a method for selectively improving the strength, toughness and fatigue resistance of a structural member in a region of high operational stress. According to one embodiment, the method includes casting the structural member in a pre-selected configuration. Alternatively, the structural member can be forged or fabricated as a wrought or machined part. Regions of the structural member having a comparatively high operational stress are identified. The structural member is secured to prevent movement. A region of the structural member having comparatively high operational stress is then mixed with a rotating friction stir welding probe to locally refine the grain structure of the structural member within the region of high operational stress to thereby improve the strength, toughness and fatigue resistance of the structural member within the region. The mixing step can include positioning a friction stir welding probe adjacent the region of the structural member having comparatively high operational stress. A rotating friction stir welding probe can then be inserted through the outer surface of the structural member proximate to the region of high operational stress to locally refine the grain structure of the high-stress region. The rotating friction stir welding probe can be moved through the structural member along a path corresponding to the region of high operational stress. The friction stir welding probe can be withdrawn from the outer surface of the structural member to thereby define a threaded opening at least partially within the region of the structural member having a locally refined grain structure. If desired, the structural member can be precipitation hardened prior to or after the inserting step.
After mixing the region of the structural member having the comparatively high operational stress, the structural member can be machined to a corresponding pre-selected shape and thickness. A threaded opening can be machined at least partially within the portion of the structural member having a locally refined grain structure. The structural member can then be secured to other structural members to form the frame of an aircraft.
Accordingly, the present invention provides an improved structural assembly and associated method of manufacture in which the assembly is constructed from structural members having enhanced strength, toughness and fatigue resistance in those regions subjected to comparatively high operational stresses. The improved structural assembly will have an increased operational life, as well as require less stock material with a corresponding decrease in the overall weight of the assembly.